Feedstock and heterogeneous structure for tough rare earth permanent magnets and production process therefor

ABSTRACT

New types of particle feedstocks and heterogeneous grain structures are provided for rare earth permanent magnets (REPMs) and their production in a manner to significantly enhance toughness of the magnet with little or no sacrifice in the hard magnetic properties. The novel tough REPMs made from the feedstock have heterogeneous grain structures, such as bi-modal, tri-modal, multi-modal, laminated, gridded, gradient fine/coarse grain structures, or other microstructural heterogeneity and configurations, without changing the chemical compositions of magnets.

RELATED APPLICATION

This application claims benefit and priority of provisional applicationSer. No. 62/707,013 filed Oct. 16, 2017, the disclosure and drawings ofwhich are incorporated herein by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with government support under Contract No.DE-AC02-07CH11358 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to particle feedstocks and torare earth permanent magnets (REPMs) made using the feedstocks with abeneficial heterogeneous grain structure as well as to magnet productionmethods. More particularly, the invention relates to particle feedstocksfor making REPM's with a heterogeneous grain structure thatsignificantly enhances mechanical properties, such as toughness and/orstrength, of the magnet with little or no sacrifice in hard magneticproperties.

BACKGROUND OF THE INVENTION

Rare-earth permanent magnets (REPMs) mainly include samarium-cobalt type(SmCo₅ and Sm₂Co₁₇ types) magnets, neodymium-iron-boron type (Nd₂Fe₁₄Btype) magnets, a neodymium-iron-carbon type magnet (R₂Fe₁₄C type, R=rareearth, La or yttrium,), a R-iron-nitrogen type magnet (R₂Fe₁₇X_(δ),R=rare earth, La or Y; X=H, C, and/or N), or a, R-iron-M-nitrogen typemagnet (R(Fe, M)₁₂X_(δ) type, R=rare earth, La or Y; M=Mo, V, Ti, Si,Al, Cr, Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N), and someother stable or metastable rare-earth-transition metal based magneticcompounds. Sm—Co and Nd—Fe—B based sintered magnets are the most commoncommercial REPMs. REPMs have excellent hard magnetic properties, such ashigh magnetocrystalline anisotropy field H_(A), high intrinsiccoercivity H_(ci), high or moderate high saturation magnetization4πM_(s), high remanence B_(r), high Curie temperatures T_(C), and highmaximum energy product (BH)_(max). For example, the commercial availableNd—Fe—B sintered magnets have an H_(A)˜67 kOe, H_(ci)˜11-30 kOe,4πM_(s)˜13-16 kGs, B_(r)˜11.6-14.4 kGs, T_(C)˜310-400° C., (BH)_(max) upto 30-52 MGOe, reversible temperature coefficient (RTC) of B_(r)calculated between 20 and 100° C.˜−0.11, maximum operation temperatureup to ˜70-200° C., respectively. The commercially available SmCo₅ (1:5)and Sm(Co,Fe,Cu,Zr)_(z) (2:17) type sintered magnets (two families ofSm—Co magnets) have an H_(A)˜400 kOe and 65 kOe, H_(ci)˜16-40 kOe,4πM_(s)˜11-13 kGs, B_(r)˜8.5-11.7 kGs, T_(C)˜681° C. and 850° C.,(BH)_(max) up to 24-35 MGOe, RTC of B_(r)˜−0.035 to −0.04, maximumoperation temperature up to 300-550° C., respectively.

REPMs have been widely used in telecommunication, consumer electronics,magnetic storage, magnetic resonance imaging (MRI), sensors, and powerand propulsion applications such as high performance motors andgenerators, wind turbine system, maglev train, ion engine and other highg-force applications, and many other applications. Nd—Fe—B magnetsexhibit the highest room temperature magnetic properties while Sm—Comagnets are the ultimate choice for high speed (30,000 to 100,000 rpm)motor applications, inertial devices such as gyroscopes andaccelerometers, and traveling wave tubes due to their excellent hardmagnetic properties and thermal stability at elevated temperatures inthe range of 200-550° C.

However, REPMs have a high-risk of mechanical failure when subjected tomechanical stress such as vibration and mechanical shock since theintermetallic compounds of Nd₂Fe₁₄B, SmCo₅ and Sm₂Co₁₇ are very brittleintrinsically with an intergranular (Nd—Fe—B) or intragranular (Sm—Co)type fracture mechanism. Nd₂Fe₁₄B, SmCo₅ and Sm₂Co₁₇ compounds have thelow symmetry tetragonal, hexagonal and rhombhedral crystal structures,respectively, that have an insufficient number of independent slipsystems. Therefore, these compounds have little or no plasticdeformation taking place before their fracture even though they haverelatively high strength.

The commercial Sm—Co and Nd—Fe—B sintered magnets are quite brittle andeasily prone to chipping, cracking or fracture in the courses of magnetmanufacture, machining, shipping, assembly, operation and applications.Machining techniques of these magnets are limited to grinding, lapping,and electric discharge machining (EDM). Even grinding and lapping thesemagnets to the required dimensions often results in chipping, cracking,and fracture, which leads to the production losses up to 30%. Thebrittleness and poor machinability of these magnets imposes seriouslimitations on the magnet shapes and uses available.

The improvement in the toughness of REPMs while maintaining their highmagnetic performance would not only improve their manufacturingefficiency and machinability, reduce part failure rate and effectivelyuse of expensive critical materials, but it would also greatly expandthe market share for this class of permanent magnets, by offeringopportunities for new applications, new shapes, and lower costs. TougherREPMs could also make it possible for production of bulky magnets witheven higher magnetic performance and larger dimensions via optimizationof alloy composition and heat treatment process, such as, higher Feconcentration and faster quench rate to increase B_(r) and (BH)_(max)values for the Sm—Co magnets.

It has been known that there was no phase transformation toughening orrising crack-growth-resistance (R-curve) behavior present in bulk REPMs.For the sintered REPMs (with the same or similar morphology, grain sizemodal, density and critical flaw size), the flexural strength σ₀ isdirectly related to fracture toughness K_(Ic) as proposed by theequation of K_(Ic)=σ₀Y√a, where Y is a dimensionless constant, whichdepends on the stress mode, shape and dimensions of the material, andgeometry and length of the crack; a is crack of length for a bodycontaining a crack. In the other words, there is a linear correlationbetween K_(Ic) and σ₀. Therefore, it is expected that improving themagnet's flexural strength indicates an improved fracture toughness, orvice versa.

Improving the mechanical properties (mainly fracture toughness andflexural strength) of REPMs is of a great scientific, technical andpractical significance. The studies on Sm—Co and Nd—Fe—B permanentmagnets have been focused on the magnetic properties for many years. Theresearch on the mechanical properties, strengthening and toughening ofthese magnets has been limitedly reported. Previous reports showed a100% improvement in fracture toughness, 69% improvement in impacttoughness, and 16% improvement in flexural strength for sintered Nd—Fe—Bmagnets through adjusting alloy compositions by the addition of smallamounts of Nd, Dy and Pr; Al, Co, Cu, Ga, Mn, Nb, Ti, V, Zr or othertransition metals, and mixtures thereof, and thus, forming fineprecipitates within the grains, reducing grain size, engineering grainboundaries and/or lattice distortion. However, the alloying processescan change the electronic, magnetic and strain energy states of thelattice, or form alternative phases with completely differentproperties, especially the addition of non-magnetic elements, and thus,the hard magnetic properties are usually degraded.

The traditional and widely used alloying method makes magnet developmenthigher cost, processing technique more complicate, and moreresource-dependent that is associated with progressive resourceexhaustion, supply uncertainty or even unavailability of criticalelements/materials. Moreover, alloyed materials with complicatedcompositions may become more difficult to recycle. Up to now, the greatchallenge on effectively resolving the brittleness problem of the REPMsstill remains.

It is known that grain refinement can make conventional ductile metalsseveral times stronger, but this comes at dramatic loss of ductility.Engineering heterogeneous microstructure of ductile metals and alloyswith multiple length scales has been applied as a strategy to enhancethe plasticity of nanostructured materials which otherwise lackextensive dislocation activity, and therefore low ductility. Forexample, heterogeneous lamella structure in Ti characterized with softmicrograined lamellae embedded in hard ultrafine-grained lamella matrixand gradient structures in Cu and steel with spatial gradients in grainsize from nanoscale to microscale typically from the surface to thecentral bulk produced an unprecedented mechanical property combination:as strong as ultrafine-grained metal and at the same time as ductile asconventional coarse-grained metal. Similarly, The formation of so calledbi-modal microstructures (e.g., consisting of a mixture of ultrafine andmicro-grains) with optimized and balanced combinations of strength andductility in Cu were also reported.

SUMMARY OF THE INVENTION

The present invention involves engineering the feedstock particle sizesand/or magnet grain sizes and subsequent grain boundary structure with afixed chemical composition to substantially tune certain mechanicalproperties of the REPMs while maintaining their high magneticperformance. The formation of such heterogeneous microstructures, suchas bi-modal, tri-modal, or multi-modal grain size structures, laminated,gridded, or gradient coarse/fine grain structures, or othermicrostructural heterogeneity and configurations, without changing thechemical compositions of magnets, is also effective to increasetoughness, as evidenced by an increased mechanical toughness propertysuch as flexural strength and/or fracture toughness, of the REPMs. Forthe REPMs, heterogeneous structures and configurations with thecombination of particle sizes or grain sizes ranging from nanometer,submicron (i.e. less than 1 micron) to several or tens of micron canhave higher strength, while comparable ductility or brittleness comparedwith the microparticle structure in the commercial REPMs that have asingle-modal coarse size of several or tens of micron made fromcommercial jet-milled precursors. The finer grains in the bi-modal,tri-modal, laminated, gridded, or gradient coarse/fine grain sizestructures and the subsequent grain boundary engineering improveespecially flexural strength and fracture toughness of the REPMs. On theother hand, the coarser grains of the microstructure have moreanti-oxidation ability and the formation of stronger texture in thesintered magnets, which can result in little or no change to the hardmagnetic properties.

Pursuant to certain embodiments of the invention, the feedstock withdifferent particle sizes or grain sizes is used to produce the sinteredREPMs with heterogeneous grain structures, such as bi-modal, tri-modal,multi-modal, laminated, gridded, or gradient coarse/fine grainstructures. In the meantime, the feedstock with different particle sizescan also effectively increase the density of the green compacts andfurther that of the sintered magnets for the sintered REPMs than thecommercial feedstock that have a single-modal micron grain size.

Practically speaking, the feedstock with a bimodal or multimodalparticle or grain sizes with average sizes in submicron scale and micronscale, respectively, are preferred. Both Sm—Co and Nd—Fe—B submicron andmicron particles have good magnetic properties and better alignment inthe magnetic field and have better resistance to oxidization duringproduction handling compared with the nanoparticles though nanoparticlesare expecting to have even higher strength and toughness.

Embodiments of the present invention thus relate to particle feedstockand heterogeneous microstructure for rare earth permanent magnets(REPMs) and their production methods. More particularly, the inventionrelates to feedstock and heterogeneous microstructure for REPMs withsignificantly enhanced toughness, while maintaining or with a minimumsacrifice in the hard magnetic properties. The novel tough REPMs haveheterogeneous grain structures, such as bi-modal, tri-modal,multi-modal, laminated, gridded, or gradient coarse/fine grainstructures, or other microstructural heterogeneity, etc., withoutchanging the chemical compositions of magnets. To increase flexuralstrength (a mechanical toughness property) of the REPMs, only particlesizes or grain sizes of feedstock are modified with fixed chemicalcompositions, rather than the conventional materials-alloying method bychanging chemical compositions and further magnet microstructure. Forpurposes of illustration and not limitation, the typical feedstockcomprises 1-99 wt. %, such as for example 5-95 wt. %, modified finer(average particle size from submicron to ˜1.5 micron) Sm—Co or Nd—Fe—Bparticle powders made from the commercial jet-milled Sm—Co or Nd—Fe—Bmicroparticle powders and 99-1 wt. %, such as for example 95-5 wt. %,commercial jet-milled Sm—Co or Nd—Fe—B microparticle powders (averageparticle size about 2-10 microns). Therefore, this invention isadvantageous in that it can be easily integrated with the currentindustry production line for sintered REPMs. The novel REPMs withheterogeneous structures and fixed chemical compositions can maintainthe hard magnetic properties, can be less dependent on the criticalelement resources, and can facilitate separation, sorting and recyclingat the end of life.

Practice of the present invention thereby provides rare earth permanentmagnets comprising a microstructure having a heterogeneous grainstructure that improves a mechanical toughness property of the magnetwith no or little reduction in magnetic properties of the magnet. TheREPMs made pursuant to embodiments of the invention can have a varietyof heterogeneous microstructures that include, but are not limited to,bi-modal, tri-modal, multi-modal, laminated, gridded, gradient grainstructures, or other microstructural heterogeneity and configurations,without changing the chemical compositions of magnets. Certainparticular embodiments of the present invention provide rare earthmagnets having a multi-modal (e.g. bi-modal, tri-modal, etc.) grainmicrostructure wherein a plurality of localized grain regions (grainclusters) having a relatively smaller average grain size are disposed inand surrounded by a matrix having relatively larger average sizes.

In one illustrative embodiment, using the feedstock developed in thisinvention, the flexural strength values of sintered Sm₂Co₁₇ typeSm₂(CoFeCuZr)₁₇ magnets made using 20 wt. %, 30 wt. % and 40 wt. %cryomilled fine powders and balance jet milled powders as feedstockswere enhanced by 50% or greater; for example, by 50% (175 MPa), 58% (185MPa) and 73% (202 MPa), respectively, relative to a flexural strengthvalue of 117 MPa for the commercial magnet. Excellent magneticproperties were maintained with the maximum energy product (BH)_(max)decreased (˜24 MGOe) by only 7.7% (less than 8%), respectively, andalmost no decrease of remanence B_(r) values. That is, practice of thepresent invention achieved significant enhancement of mechanicaltoughness properties, such as flexural strength, with no or littledegradation of the hard magnetic properties; e.g. (BH)_(max) and B_(r),such as less than 10% degradation of (BH)_(max) and B_(r) at roomtemperature (20° C.).

In another embodiment of the invention, using the feedstock developed inthis invention, the flexural strength values of sintered SmCo₅ typemagnets made using 20 wt. % and 30 wt. % cryomilled fine powders andbalance jet milled powders as feedstocks, respectively, were enhanced by37% (214 MPa) and 21% (189 MPa) relative to a flexural strength value of156 MPa for the commercial magnet. The values of both B_(r) and(BH)_(max) of the magnets made with the modified microstructurespursuant to the invention were even higher than those of the commercialcounterpart magnet. That is, (BH)_(max) increased by 10.5% and 15.8%,B_(r) increased by 6.9% and 8.0% for these two magnets made pursuant tothe invention, respectively.

In the two embodiments just discussed, the Sm—Co bulk sintered magnetshave a heterogeneous bi-modal grain microstructure without changing thechemical compositions of magnets. Other heterogeneous grain structures,such as tri-modal grain structure, multi-modal grain structure, finelaminated grain/coarse grain structure, gridded fine/coarse grainstructure (i.e. square honeycomb fine/coarse grain structure), gradientgrain structures, (such as having larger grains progressing to smallergrains from one side to another side across the microstructure, orhaving larger grains progressing to smaller grains that reside in thecentral areas and then progressing to larger grains, from one side toanother side across the microstructure, or verse versa), or othermicrostructural heterogeneity and configurations, can also be producedin the REPMs using the feedstock and production process thereofdeveloped in this invention. On the other hand, the heterogeneousstructures can also be introduced into the REPMs by other fabricationmethods including, but not limited to, friction consolidation extrusion,3D printing, surface mechanical attrition treatment (SMAT), equalchannel angular extrusion (ECAE), hot accumulative roll bonding (ARB),hot asymmetric rolling, high pressure torsion (HPT), hot drawing,mechanical milling, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows morphology (SEM images) of SmCo₅ powders (1500×) (Jetmilled powders shown in top row) and further cryomilled in LN₂ for 5 hr(shown in bottom row), according to one embodiment of the invention. Theright column is the corresponding enlarged images (5000×).

FIG. 2 shows grain size distribution of the SmCo₅ powders (Jet milledpowders (shown in top graph) and further cryomilled in LN₂ for 5 hr(shown bottom graph) with mean sizes of about 1.5 μm and 2.0 μm,respectively. These results were obtained from the SEM images analyzedby the image J software.

FIG. 3 presents demagnetization curves of 2:17 type Sm₂(CoFeCuZr)₁₇sintered magnets made from feedstock of (100−x) wt. % 2:17 type jetmilled (JM) powders+x wt. % 2:17 type cryomilled in liquid nitrogen (CM)powders, x=0, 10, 15, 20, 25, 30, 40, according to an embodiment of theprocess of the invention.

FIG. 4 illustrates demagnetization curves of 1:5 type Sm—Co sinteredmagnets made from feedstock of (100−x) wt. % 1:5 type jet milled (JM)Sm—Co powders+x wt. % 1:5 type cryomilled in liquid nitrogen (CM)powders, x=0, 10, 15, 20, 25, 30, 40, in accordance with an embodimentof the process of the invention.

FIG. 5 shows an optical photomicrograph (200×) of cross-sectionmicrostructure of the 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets madefrom 100 wt. % 2:17 type jet milled (JM) powders. Typical single-modalgrain size structure was observed. The specimen was mechanicallypolished then etched with 2% nital etchant for the metallographicexamination.

FIG. 6 shows optical photomicrographs (500×) of cross-sectionmicrostructure from selected areas 1 and 2 as shown in FIG. 5, for the2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets made from 100 wt. % 2:17 typejet milled (JM) powders. Typical single-modal grain size structure wasobserved.

FIG. 7 shows grain size distribution of the 2:17 type Sm₂(CoFeCuZr)₁₇sintered magnets made from 100 wt. % 2:17 type jet milled (JM) powders.Typical single-modal grain size structure was observed with a mean grainsize of about 40 μm.

FIG. 8 shows optical photomicrograph (200×) of cross-sectionmicrostructure of the 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets madefrom the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt.% 2:17 type cryomilled in liquid nitrogen (CM) powders. The specimen wasmechanically polished then etched with 2% nital etchant for themetallographic examination. A typical bio-modal grain size structure wasobserved in the sintered magnet. The fine grains (FG) formed clusterareas (marked by white ovals) those uniformly distributed within thecoarse grain (CG) matrix. The Sm—Co sintered magnet with a 3D gradientharmonic structure of controlled bimodal grain size distributionexhibited considerably higher flexural strength and comparable magneticproperties, compared with the magnet with a single-modal grainedstructure as shown in FIGS. 5-7.

FIG. 9 shows optical photomicrograph (200×) of cross-sectionmicrostructure of the 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets madefrom the feedstock of 70 wt. % 2:17 type jet milled (JM) powders+30 wt.% 2:17 type cryomilled in liquid nitrogen (CM) powders. The areas 1, 2,3 and 4 marked by squares were selected for further enlargedobservation.

FIG. 10 shows optical photomicrographs (500×) of cross-sectionmicrostructure from selected areas 1, 2, 3 and 4 as shown in FIG. 9, forthe 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets made from the feedstockof 70 wt. % 2:17 type jet milled (JM) powders+30 wt. % 2:17 typecryomilled in liquid nitrogen (CM) powders. A typical bio-modal grainsize structure was observed in the sintered magnet. The fine grains (FG)formed cluster areas (marked by white ovals) those uniformly distributedwithin the coarse grain (CG) matrix.

FIG. 11 shows optical photomicrographs (1000×) of the cross-sectionmicrostructure from selected areas for the 2:17 type Sm₂(CoFeCuZr)₁₇sintered magnets made from the feedstock of 70 wt. % 2:17 type jetmilled (JM) powders+30 wt. % 2:17 type cryomilled in liquid nitrogen(CM) powders. A typical bio-modal grain size structure was observed inthe sintered magnet. The fine grains (FG) formed cluster areas (markedby white ovals) in FIG. 10 and uniformly distributed within the coarsegrain (CG) matrix.

FIGS. 12a and 12b show grain size distribution of fine grain areas (FGareas, Fi. 12 b) and coarse grain matrix (CG areas, FIG. 12a ) of 2:17type Sm₂(CoFeCuZr)₁₇ sintered magnets made from the feedstock of 70 wt.% jet milled (JM) powders+30 wt. % cryomilled in liquid nitrogen (CM)powders. The mean sizes were about 24 μm and 6 μm for the FG areas andCG matrix, respectively. These results were obtained from the opticalimages analyzed by the image J software.

FIG. 13 shows scanning electron microscope (SEM) images of the fracturesurface of Sm₂(CoFeCuZr)₁₇ type sintered magnets made from raw materialsof (100−x) wt. % 2:17 type jet milled powders+x wt. % 2:17 typecryomilled powders. Right column shows the enlarged images from theselected areas (marked by the black-line ovals). Top images: x=0 wt. %sample, a single-modal coarse grain size (˜40 μm) microstructure; andBottom images: x=30 wt. % sample having a 3D gradient harmonicmicrostructure with a bi-modal grain size distribution. A higher densitywhile smaller size of river patterns and cleavage steps were observed inthe heterogeneous magnets (bottom) with a bi-modal grain sizedistribution compared with the x=0 wt. % sample with a single-modalcoarse grain size (top).

FIG. 14 shows SEM images of the fracture surface of Sm₂(CoFeCuZr)₁₇ typesintered magnets made from raw materials of 70 wt. % jet milledpowders+30 wt. % cryomilled powders. Bottom-left and top-right imagesare the enlarged images from the selected areas: area 1—fine grain (FG)area, and area 2—coarse grain (CG) matrix (marked by the black-lineovals). A higher density while smaller size of river patterns andcleavage steps were observed in the heterogeneous magnets, especially inthe finer grain cluster regions on the fracture surface.

Table 1 lists the flexural strength of sintered 2:17 type and 1:5 typeSm—Co magnets made from feedstock of (100−x) wt. % 2:17 or 1:5 type jetmilled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled in liquidnitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40, according to theembodiment of the invention.

Table 2 lists magnetic properties and density of sintered 2:17 type and1:5 type Sm—Co magnets made from feedstock of (100−x) wt. % 2:17 or 1:5type jet milled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled inliquid nitrogen (CM) powders, x=0, 10, 15, 20, 25, 30, 40.Remanence—B_(r), intrinsic coercivity—H_(ci), maximum energyproduct—(BH)_(max), value of H_(c) at 0.9B_(r)−H_(k), coercivity—H_(c)magnet density—ρ.

Table 3 lists Mean grain sizes of the fine grain (FG) areas and thecoarse grain (CG) matrix, and overall mean gain sizes for the 2:17 typeSm₂(CoFeCuZr)₁₇ sintered magnets made from the feedstock of (100−x) wt.% jet milled (JM) powders+x wt. % (x=0, 10, 15, 20, 30, 40 wt. %)cryomilled (CM) powders.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to rare earth permanent magnets (REPMs)having a heterogeneous microstructure and their production in a mannerto provide significantly enhanced toughness; i.e. resistance-to-fractureas evidenced by enhanced mechanical toughness property such as flexuralstrength and/or fracture toughness, while maintaining or with a minimumsacrifice in the hard magnetic properties. The REPMs made pursuant toembodiments of the invention have heterogeneous grain microstructures,such as bi-modal, tri-modal, multi-modal, laminated, gridded, orgradient coarse/fine grain structures, or other microstructuralheterogeneity, etc., without the need for changing the chemicalcompositions of magnets. To increase flexural strength and/or fracturetoughness of the REPMs, particle sizes or grain sizes of the particlefeedstock are modified with fixed chemical feedstock compositions inthis invention. For purposes of illustration and not limitation thetypical feedstock comprises 1-99 wt. % modified finer (average particlesize from less than 1 micron; i.e. submicron to about 1.5 micron) Sm—Coor Nd—Fe—B particle powders made from the commercial jet-milled Sm—Co orNd—Fe—B microparticle powders and 99-1 wt. % commercial jet-milled Sm—Coor Nd—Fe—B microparticle powders (average particle size about 2-10micron). As a result, practice of the invention can be easily integratedwith the current industry production line for sintered REPMs. In certainembodiments, the powders can be mixed together under argon or othernon-reactive atmosphere in a mixer or mill for greater than 0 to 100hours or more as needed. The powders can be formed into a green compactand consolidated by techniques that include, but are not limited to,powder metallurgy processing, hot pressing, friction consolidationextrusion, 3D printing, surface mechanical attrition treatment (SMAT),equal channel angular extrusion (ECAE), hot accumulative roll bonding(ARB), hot asymmetric rolling, high pressure torsion (HPT), hot drawing,and mechanical milling. The REPMs with heterogeneous microstructures andfixed chemical compositions pursuant to embodiments of the invention canmaintain the hard magnetic properties without substantial degradation ofthe hard magnetic properties such as BH_(max) and B_(r), can be lessdependent on the critical element resources, and can facilitateseparation, sorting and recycling at the end of life.

The following examples are offered for purposes of further illustration,but not limitation, with respect to the present invention:

Fine Sm—Co (both 2:17 and 1:5 types) powders (average particle sizeabout 1 μm) were synthesized by cryomilling in liquid nitrogen (LN₂),using commercial jet milled microparticle powders as the precursors anda SPEX 6875D Freezer/Mill. FIGS. 1 and 2 show typical morphology andgrain size distribution of jet milled SmCo₅ powders with/without (w/o)further ball milling in LN₂ for 5 hrs. The jet milled precursor powderswere mainly composed of particles with a size of about 1.0-2.5 μm with amean particle size of about 2.0 μm. The particle size range was withinabout 0.7-8.0 μm. With increasing the cryomilling time from 1 hr to 5hrs, the average particle size continuously decreased. The Sm—Co powdersafter cryomilled for 5 hrs were mainly composed of particles with a sizeof about 1.0 μm or less, which had an average particle size of about 1.5μm. The particle size range was within about 0.5-6.0 μm. Similar resultswere obtained for the 2:17 type Sm₂(CoFeCuZr)₁₇ jet milled andcryomilled powders. The jet milled Sm₂(CoFeCuZr)₁₇ powders were composedof irregular microparticles with a particle size mainly in the range ofabout 0.7-3.0 μm and a mean particle size of 2.3 μm. The overallparticle size range of the jet milled powders was within about 0.7-8.0μm. Whereas, the Sm₂(CoFeCuZr)₁₇ powders further cryomilled for 2 hrswere composed of finer irregular microparticles with a particle sizemainly in the range of about 0.5-1.5 μm and a mean particle size of 1.3μm and less-sharp edges. The overall particle size range of thecryomilled powders was within about 0.5-8.0 μm. Both the conventionaljet milled microparticles and the further cryomilled finer particles hada single-crystal structure. These particle size results were obtainedfrom the SEM images analyzed by image J software. Both the cryomilledfiner particles and the commercial jet milled microparticles were mainlysingle-crystal structure. Besides cryomilling in liquid nitrogen, theother finer powder preparation methods wherein the particle size rangingfrom nanometer, submicron, and micron scale or their mixtures, that issmaller than that of jet-milled powders with a typical average particlesize of about 2-5 micron, include but are not limited to, some top-downand bottom-up approaches, such as, multiple jet milling, low or highenergy ball milling at room temperature in inert gas (Ar, N₂, or He) orin solvent media (ethanol, hexane, heptane, toluene, etc.), organicsurfactant-assisted high energy ball milling at room temperature, inertgas atomization, gas condensation, spark erosion, chemicalprecipitation, sol-gel, pyrolysis and hydrothermal synthesis, plasmaarcing, chemical vapor deposition (CVD), physical vapor deposition(PVD), electrodeposition, atomic layer deposition (ALD), etc.

The cryomilled Sm—Co sub-micron powders produced pursuant to embodimentsof this invention were then mixed with the jet milled precursor powdersunder an argon atmosphere in a SPEX 8000M Mixer/Mill without any millingballs for a time of 2 or 3 minutes, which more generally can be up to 15minutes or more or other suitable blending time. The particle mixturethen is subjected to conventional powder metallurgy method (i.e.sintering a pressed compact) to produce a bulk magnet with grainboundary engineering or modified microstructure. The particularillustrative powder metallurgy steps typically include cold compactionof the magnetically aligned powder mixture to form a green compact andthen sintering the green compact, although the powders can be formedinto a green compact and consolidated by techniques that include, butare not limited to, powder metallurgy processing, hot pressing, frictionconsolidation extrusion, 3D printing, surface mechanical attritiontreatment (SMAT), equal channel angular extrusion (ECAE), hotaccumulative roll bonding (ARB), hot asymmetric rolling, high pressuretorsion (HPT), hot drawing, and mechanical milling. The powdermetallurgy method optionally can include preparation of ingot chips bystrip casting or bulk ingot by induction melting or arc melting,hydrogen decrepitation or crushing into coarse powders of about 100micron or less sizes, jet milling or ball milling into finemicroparticles of the average particle sizes described above,magnetically aligning by a 4 or 7 Tesla pulsed magnetic field andpre-pressing powder mixtures into green compacts by a pressure of 35,000psi (about 241 MPa) using a Nikisso CL15-45-30 iso-static press, andsubsequent heat treatment procedure, including sintering, solution,temper, and aging.

In the examples below, the green compacts were pre-pressed by a pressureof 241 MPa using the above Nikisso CL15-45-30 iso-static press andsintered. The 2:17 type Sm₂(CoFeCuZr)₁₇ were sintered at 1190-1220° C.for 1-2 hrs, solution tempered at 1150-1185° C. for 1-7 hrs, and aged at800-850° C. for 5-10 hrs then cooling to 400° C. at a ramp rate of0.7-1.0° C./min., further aging at 400° C. for 1-10 hrs. Sintered 1:5type SmCo₅ were sintered at 1130-1150° C. for 1-2 hrs, solution temperedat 850-900° C. for 5-7 hrs, and then quenched in argon.

Sintered 2:17 type Sm₂(CoFeCuZr)₁₇ magnet density ρ was 8.4 g/cc, whichwas about 99% of theoretical value. Demagnetization curves and magneticproperties are shown in FIG. 3 and Table 2 for the sintered 2:17 typemagnets made from feedstock of (100−x) wt. % 2:17 type jet milled (JM)powders+x wt. % 2:17 type cryomilled in liquid nitrogen (CM) powders.The remanence B_(r) was about 10.6 kGs, and intrinsic coercivity H_(ci)was higher than 24 kOe for all the 2:17 type magnets. Maximum energyproduct (BH)_(max) was about 26 MGOe for x=0 sample while its valuedecreased a little with increasing of x values.

Table 1 below lists the flexural strength of sintered 2:17 type and 1:5type Sm—Co magnets made from feedstock of (100−x) wt. % 2:17 or 1:5 typejet milled (JM) powders+x wt. % 2:17 or 1:5 type cryomilled in liquidnitrogen (CM) powders. By engineering the grain size and grain-boundarymicrostructure, the flexural strength of the sintered Sm₂Co₁₇ typemagnets were enhanced by 50%, 58%, and 73% (175 MPa, 185 MPa and 202 MPafor the samples with x=20, 30, and 40, respectively) compared to 117 MPafor the sample with x=0. A flexural strength value of 120 MPa for thesample with x=0 was reported for a commercial Sm₂Co₁₇ type magnets(http://www.electronenergy.com/products/materials). Excellent magneticproperties were maintained with the maximum energy product (BH)_(max)(˜24 MGOe) decreased by only 7.7% (less than 8%), respectively, andalmost no decrease of remanence B_(r) values. Flexural strengthsreported above were measured using the 3-point bending ASTM flexure testno. C1161-13. Fracture toughness, another mechanical toughness property,can be measured by the Charpy V-notch or IZOD ASTM tests no. C1421-16.

FIG. 4 and Table 2 below show demagnetization curves and magneticproperties of the sintered 1:5 type SmCo₅ magnets made from feedstock of(100−x) wt. % 1:5 type jet milled (JM) powders+x wt. % 1:5 typecryomilled in liquid nitrogen (CM) powders. By engineering the grainsize and grain-boundary microstructure, the flexural strength of newsintered SmCo₅ type magnets were enhanced by 37% and 21%, which was 214MPa and 189 MPa for the samples with x=20 and 30, respectively, comparedto a flexural strength value of 156 MPa for the sample with x=0. Aflexural strength value of 130 MPa was reported for commercial SmCo₅magnets (http://www.electronenergy.com/products/materials). The higherflexural strength value of 156 MPa for the present sample with x=0 thanthat of the reported commercial magnet was likely due to the variableparticles sizes of feedstocks and thus grain sizes of the sinteredmagnets. The values of both remanence B_(r) and the maximum energyproduct (BH)_(max) of the samples with modified microstructures pursuantto the invention were even higher than those of the commercialcounterpart x=0 sample. That is, (BH)_(max) (˜21 and 22 MGOe) increasedby 10.5% and 15.8%, B_(r) (˜9.3 and 9.4 KG) increased by 6.9% and 8.0%for the samples with x=20 and 30, respectively.

With respect to sintered microstructures, a typical single-modal grainsize structure with a mean grain size of about 40 μm was observed in the2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets made from 100 wt. % 2:17 typejet milled (JM) powders, as shown in FIGS. 5-7. In contrast, the Sm—Cobulk magnets made from the feedstock mixtures pursuant to the inventionhave a bi-modal grain size microstructure, as shown in the typical FIGS.8-12 a, 12 b. The fine grains (FG) formed cluster areas (localizedregions) and uniformly distributed within the coarse grain (CG) matrix.FIGS. 12a, 12b show grain size distribution of fine grain areas (FG) andcoarse grain (CG) matrix of 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnetsmade from the feedstock of 70 wt. % jet milled (JM) powders+30 wt. %cryomilled in liquid nitrogen (CM) powders. The mean sizes were about 6μm and 24 μm for the FG areas and CG matrix, respectively. The sizes ofthe FG localized areas were in the range of about 60-190 μm. The otherSm—Co sintered magnets developed by practice of embodiments of thepresent invention showed similar heterogeneous microstructure with abi-modal grain size distribution. Table 3 lists mean grain sizes of thefine grain (FG) areas and the coarse grain (CG) matrix, and overall meangain sizes for the 2:17 type Sm₂(CoFeCuZr)₁₇ sintered magnets made fromthe feedstock of (100−x) wt. % jet milled (JM) powders+x wt. % (x=0, 10,15, 20, 30, 40 wt. %) cryomilled (CM) powders. With increasing x values,the mean grain sizes of both FG areas and CG matrix, and overall meangain sizes decreased monotonously. These results were obtained from theoptical images analyzed by image J software. This novel 3D gradientharmonic microstructure with a bi-modal grain size resulted inconsiderably higher flexural strength and higher fracture toughness, andcomparable magnetic properties, compared with the magnet with asingle-modal grain sized microstructure as shown in FIGS. 5-7.

The enhancement of flexural strength of Sm—Co sintered magnets resultedfrom the grain size refinement with the contributions from bothlocalized fine grain cluster regions and a general grain size reductionfrom the coarse grain matrix, i.e. bimodal grain size engineering. Thelocalized fine grain clusters are more effective in preventing cracknucleation or propagation via acting as strengthening sites andtherefore they have higher flexural strength. Since it is known that thesintered Sm—Co magnets have a brittle intragranular cleavage fracturemechanism under normal stress, the characteristic morphology of cleavagefracture, such as river patterns and cleavage steps, was observed on thefracture surface of the Sm₂(CoFeCuZr)₁₇ type sintered magnets as shownin FIGS. 13 and 14. No section shrink, fiber region, or shear lip wasobserved on the fracture surface. A higher density while smaller size ofriver patterns and cleavage steps were observed in the fine grainclusters on the fracture cross-section surface as shown in FIGS. 13 and14, which increased the energy needed for nucleation or propagation ofthe main crack. This is a structure proof of the strengthening effectfrom the fine grain clusters.

The magnets developed pursuant to embodiments of the invention can beexpected to find similar applications in various industries as those ofcommercial sintered or die-upset REPMS. Applications include, but arenot limited to, e.g., telecommunication, magnetic storage, biomedicalequipment, consumer electronics, sensors, power and propulsionapplications such as high performance motors and generators and ionengines, inertial devices such as gyroscopes and accelerometers, andtraveling wave tubes, and many more.

Tables 1, 2 and 3 appear below:

TABLE 1 Flexural strength of sintered 2:17 type and 1:5 type Sm—Comagnets made from feedstock of (100 − x) wt. % 2:17 or 1:5 type jetmilled (JM) powders + x wt. % 2:17 or 1:5 type cryomilled in liquidnitrogen (CM) powders, x = 0, 10, 15, 20, 25, 30, 40. CM/JM weight ratioFlexural Increase Samples x/(100 − x) strength (MPa) STD by (%) SmCo₅ 0/100 156 (130*) 14 — 10/90 183 6 17 15/85 176 4 13 20/80 214 2 3725/75 201 9 29 30/70 189 14 21 40/60 182 5 17 Sm₂(CoFeCuZr)₁₇  0/100 117(120*) 3 — 10/90 159 10 36 15/85 161 17 38 20/80 175 20 50 30/70 185 2558 40/60 202 10 73 Note: Values marked as * were from the commercialSm—Co magnets (http://www.electronenergy.com/products/materials).

TABLE 2 Magnetic properties and density of sintered 2:17 type and 1:5type Sm—Co magnets made from feedstock of (100 − x) wt. % 2:17 or 1:5type jet milled (JM) powders + x wt. % 2:17 or 1:5 type cryomilled inliquid nitrogen (CM) powders, x = 0, 10, 15, 20, 25, 30, 40. Remanence -B_(r), intrinsic coercivity - H_(ci), maximum energy product -(BH)_(max), value of H_(c) at 0.9B_(r) - H_(k), coercivity - H_(c),density - ρ. x/(100 − x) B_(r) H_(ci) (BH)_(max) H_(k) H_(c) ρ Samplesratio (kG) (kOe) (MGOe) (kOe) (kOe) (g/cc) SmCo₅  0/100 8.7 >24 19 22.08.6 8.1 10/90 9.1 >24 21 21.3 9.0 8.4 15/85 9.1 >23 21 20.9 9.0 8.320/80 9.3 >23 21 20.7 9.1 8.3 25/75 9.4 >23 22 20.2 9.3 8.5 30/709.4 >23 22 15.5 9.2 8.5 40/60 9.4 18.7 22 12.6 9.1 8.5 Sm₂(CoFeCuZr)₁₇ 0/100 10.6 >24 26 12.0 9.8 8.4 10/90 10.6 >24 26 11.5 9.7 8.4 15/8510.6 >24 26 10.1 9.6 8.4 20/80 10.6 >24 26 8.8 9.5 8.4 25/75 10.6 >24 257.6 9.3 8.4 30/70 10.6 >24 24 5.9 9.0 8.4 40/60 10.9 >23 24 4.2 8.7 8.4

TABLE 3 Mean grain sizes of the fine grain (FG) areas and the coarsegrain (CG) matrix, and overall mean gain sizes for the 2:17 typeSm₂(CoFeCuZr)₁₇ sintered magnets made from the feedstock of (100 − x)wt. % jet milled (JM) powders + x wt. % (x = 0, 10, 15, 20, 30, 40 wt.%) cryomilled (CM) powders. CM/JM powder Mean Mean grain weight grainsize Overall ratio size from CG mean x/(100 − from FG matrix grain sizeD Samples x) % areas (μm) (μm) (μm) Sm₂(CoFeCuZr)₁₇  0/100 — 40 40 10/9014 29 28 15/85 12 26 24 20/80 8 25 22 30/70 6 24 19 40/60 5 12 9

The present invention can be practiced to make rare earth permanentmagnets that include, but are not limited to, a samarium-cobalt type(SmCo₅ and Sm₂Co₁₇ types) magnet, a neodymium-iron-boron type (Nd₂Fe₁₄Btype) magnet, a neodymium-iron-carbon type magnet (R₂Fe₁₄C type, R=rareearth, La or yttrium,), a R-iron-nitrogen type magnet (R₂Fe₁₇X_(δ) type,R=rare earth, La or Y; X=H, C, and/or N), or a R-iron-M-nitrogen typemagnet (R(Fe, M)₁₂X_(δ) type, R=rare earth, La or Y; M=Mo, V, Ti, Si,Al, Cr, Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N). Themagnets also can comprise a stable or metastable rare-earth-transitionmetal based magnetic compound, having the formula of R₂TM₁₄A, RTM₅,RT₂M₁₇, R₂TM₁₇A, RTM₇, RTM₇A, RTM₁₂, RTM₁₂A, R₃TM₂₉, and R₃TM₂₉A,wherein R is one or a combination of rare earths, La or yttrium, TM isone or a mixture of transition metals, A is one or a combination of thefollowing elements: Be, B, C, N, S, Mg, Al, Si, P, Ga, Ge, As, Se, In,Sn, Sb, Te, I, Pb, and Bi.

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While the exemplary embodiments of the present invention have been shownand described herein, it will be apparent to those skilled in the artthat many changes and modifications may be made without departing fromthe invention in its true scope and broader aspects. The appended claimsare therefore intended to cover all such changes and modifications asfall within the spirit and scope of the invention as set forth in thefollowing claims.

What is claimed is:
 1. A rare earth permanent magnet having amicrostructure characterized by a heterogeneous grain structure thatimproves a mechanical property of the magnet.
 2. The magnet of claim 1wherein the mechanical property is fracture toughness and/or mechanicalstrength.
 3. The magnet of claim 2 that exhibits a flexural strengthincrease of 50% or greater at 20° C. with little or no reduction of(BH)_(max) and B_(r) magnetic properties.
 4. The magnet of claim 1wherein the heterogeneous grain structure comprises a plurality oflocalized grain regions having relatively smaller average grain sizedisposed in and surrounded by a matrix comprising relatively largeraverage grain size.
 5. The magnet of claim 1 wherein the heterogeneousgrain structure comprises a bi-modal grain size distribution.
 6. Themagnet of claim 1 wherein the heterogeneous grain structure comprises atri-modal or multi-modal grain size distribution.
 7. The magnet of claim1 wherein the heterogeneous grain structure comprises a gradientdistribution of grain sizes across the microstructure.
 8. The magnet ofclaim 1 comprising a samarium-cobalt type (SmCo₅ and Sm₂Co₁₇ types)magnet, a neodymium-iron-boron type (Nd₂Fe₁₄B type) magnet, aneodymium-iron-carbon type magnet (R₂Fe₁₄C type, R=rare earth, La oryttrium,), a R-iron-nitrogen type magnet (R₂Fe₁₇X_(δ) type, R=rareearth, La or Y; X=H, C, and/or N), or a R-iron-M-nitrogen type magnet(R(Fe, M)₁₂X_(δ) type, R=rare earth, La or Y; M=Mo, V, Ti, Si, Al, Cr,Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N).
 9. The magnet ofclaim 1 comprising a stable or metastable rare-earth-transition metalbased magnetic compounds, having the formula of R₂TM₁₄A, RTM₅, RT₂M₁₇,R₂TM₁₇A, RTM₇, RTM₇A, RTM₁₂, RTM₁₂A, R₃TM₂₉, and R₃TM₂₉A, wherein R isone or a combination of rare earths, La or yttrium, TM is one or amixture of transition metals, A is one or a combination of the followingelements: Be, B, C, N, S, Mg, Al, Si, P, Ga, Ge, As, Se, In, Sn, Sb, Te,I, Pb, and Bi.
 10. The magnet of claim 1 which comprises consolidatedpowders.
 11. The magnet of claim 9 which is sintered.
 12. Feedstockcomprising a mixture of rare earth-bearing powders wherein a selectedpercentage of the powders has relatively smaller average particle sizeranging from less than 1 micron to about 1.5 microns with a remainingbalance of the powders of the mixture having relatively larger averageparticle size, wherein the mixture forms a heterogeneous grain structurewhen the mixture is consolidated as a rare earth permanent magnet. 13.The feedstock of claim 12 which forms a permanent magnet having theheterogeneous grain structure that comprises a plurality of localizedgrain regions having relatively smaller average grain size disposed inand surrounded by a matrix comprising relatively larger average grainsize.
 14. The feedstock of claim 12 which forms a magnet having theheterogeneous grain structure that comprises a bi-modal grain sizedistribution.
 15. The feedstock of claim 12 which forms theheterogeneous grain structure that comprises a tri-modal or multi-modalgrain size distribution.
 16. The feedstock of claim 12 which forms amagnet having the heterogeneous grain structure that comprises a griddeddistribution of grain sizes in the microstructure.
 17. The feedstock ofclaim 12 which forms a magnet having the heterogeneous grain structurethat comprises a gradient distribution of grain sizes in themicrostructure.
 18. The feedstock of claim 12 wherein the rareearth-bearing powders are selected from at least one of asamarium-cobalt type (SmCo₅ and Sm₂Co₁₇ types) magnet; aneodymium-iron-boron type (Nd₂Fe₁₄B type) magnet; aneodymium-iron-carbon type magnet (R₂Fe₁₄C type, R=rare earth, La oryttrium,); a R-iron-nitrogen type magnets (R₂Fe₁₇X_(δ) type, R=rareearth, La or Y; X=H, C, and/or N); a R-iron-M-nitrogen type magnet(R(Fe, M)₁₂X_(δ) type, R=rare earth, La or Y; M=Mo, V, Ti, Si, Al, Cr,Cu, Ga, Ge, Mn, Nb, Sn, Ta, W or Fe; X=H, C, and/or N); and a stable ormetastable rare-earth-transition metal based magnetic compound havingthe formula of R₂TM₁₄A, RTM₅, RT₂M₁₇, R₂TM₁₇A, RTM₇, RTM₇A, RTM₁₂,RTM₁₂A, R₃TM₂₉, or R₃TM₂₉A, wherein R is one or a combination of rareearths or yttrium, TM is one or a mixture of transition metals, A is oneor a combination of the following elements: Be, B, C, N, S, Mg, Al, Si,P, Ga, Ge, As, Se, In, Sn, Sb, Te, I, Pb, and Bi.
 19. A method ofblending the feedstock of claim 12 wherein the relatively smallerparticles are cryomilled in liquid nitrogen for a time using jet milledmicroparticle powders as the precursor powders.
 20. The method of claim19 wherein the relatively smaller particles are blended with commercialjet-milled powders with the relatively smaller particles being presentin an amount greater than 1% to 99% by weight of the mixture.
 21. Themethod of claim 20 wherein the blending is conducted in argon or otherinert or non-reactive gas for a time from greater than 0 to 100 hrs ormore blending time.
 22. A method of producing a rare earth permanentmagnet that possesses flexural strength increased by 50% or above atroom temperature (20° C.), said method comprising the steps of: (a)preparing the feedstock of claim 12, and (b) consolidating the feedstockto form a rare earth permanent magnet having a microstructure with aheterogeneous grain structure that increases the flexural strength ofthe magnet.
 23. The method of claim 22 that produces a microstructurehaving a plurality of localized grain regions having relatively smallergrain size that are disposed in and surrounded by a matrix comprisingrelatively larger grain size.
 24. The method of claim 22 wherein theconsolidating step includes powder metallurgy processing, hot pressing,friction consolidation extrusion, 3D printing, surface mechanicalattrition treatment (SMAT), equal channel angular extrusion (ECAE), hotaccumulative roll bonding (ARB), hot asymmetric rolling, high pressuretorsion (HPT), hot drawing, and/or mechanical milling.